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FreeBSD kernel development setup

Table of Contents

Demonstration
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Introduction
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I’m trying to learn more about how FreeBSD works.

There is a lot of hidden magic that kernel does which I would like to witness. To help myself understand what is going on, I hope to build tools that will help me get useful information about what is happening within the kernel.

I’m a newbie to all of this and I’m trying to document everything I did. Hopefully this blog can help someone to get started.

Please contact me if you have any feedback. :)

Who is this blog post for?
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  1. Myself.
  2. People who want to start with FreeBSD development but don’t know how.

Do I need a beefy computer for this?
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Your computer doesn’t need to be something super fast. However, I’d recommend something that can compile FreeBSD source code decently fast so you don’t have to wait a lot.

Compilation time depends on many factors, such as:

  • your hardware
  • what you’re building
  • if you’ve changed a commonly included header file

With 7800X3D, a full rebuild of 16-CURRENT kernel on 15.0-RELEASE takes about 2 minutes. Building and installing the kernel when only one .c source file has changed takes about 2.5 seconds on the same machine.

VM setup
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We’ll use 9P protocol to share the filesystem between VM and the host. This makes installing software and running the VM super simple.

I will be using ZFS - so I will create a dataset to hold the VM:

zfs create -o mountpoint=/lab zroot/lab
zfs create zroot/lab/vm0

Fetch the necessary files:

cd /lab
fetch https://download.freebsd.org/snapshots/amd64/16.0-CURRENT/base.txz
fetch https://download.freebsd.org/snapshots/amd64/16.0-CURRENT/kernel.txz

Installing the VM:

tar -C vm0 base.txz
tar -C vm0 kernel.txz

Adding the necessary configuration:

cat <<EOF > vm0/boot/loader.conf
virtio_p9fs_load="YES"
vfs.root.mountfrom="p9fs:root"
EOF
echo 'hostname="freebsd"' > vm0/etc/rc.conf
echo 'root / p9fs rw 0 0' > vm0/etc/fstab
# if you want the VM to auto-login as root
sed -I'' 's|:np:nc:sp#0:|:al=root:np:nc:sp#0:|g' vm0/etc/gettytab

Create a snapshot with the configured VM:

zfs snap zroot/lab/vm0@base

Load and run the VM:

bhyveload -m 1G -h vm0 vm0 && bhyve -H -m 1G -A -G localhost:1234 -l com1,stdio -s 0,hostbridge -s 3,virtio-9p,root=vm0 -s 31,lpc vm0

Your VM is now all set up! :)

Making changes to the kernel
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Getting FreeBSD-src
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Clone the official FreeBSD source repository:

zfs create zroot/lab/src
zfs create zroot/lab/obj
git clone --depth 1 ssh://anongit@git.FreeBSD.org/src.git src

Building the kernel
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MAKEOBJDIRPREFIX=/lab/obj make -C src -j $(sysctl -n hw.ncpu) -ss buildkernel
# this could take a while depending on your hardware

Installing the kernel
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MAKEOBJDIRPREFIX=/lab/obj make -C src -j $(sysctl -n hw.ncpu) -ss DESTDIR=/lab/vm0 installkernel

Testing
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Start the VM and check which kernel it is running:

bhyveload -m 1G -h vm0 vm0 && bhyve -H -m 1G -A -G localhost:1234 -l com1,stdio -s 0,hostbridge -s 3,virtio-9p,root=vm0 -s 31,lpc vm0
# run this inside the VM once it's up:
uname -a

Attach a debugger
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# from the src directory
kgdb -r localhost:1234 /lab/obj/lab/src/amd64.amd64/sys/GENERIC/kernel

Homework
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Reading:

  • read man 7 build.

Practice:

  • Set a breakpoint on function ‘strlen’ and continue execution.
  • Enter an empty command to the shell. (Press enter.)
  • Check why ‘strlen’ is getting called.

Hint: print (char const *)$rdi